Safety relay for guarding circuits — SIL 3 / PL e
The Siemens 3SK1120-2AB40 is a SIRIUS safety relay with solid-state enabling circuits, rated for 24 V DC. It carries SIL 3 per IEC 61508 and PL e per ISO 13849-1, meaning it's certified for safety functions up to the highest performance level used in machine guarding, emergency stop, and light-curtain circuits. The 17.5 mm width and screw-and-snap-on mounting fit standard DIN-rail enclosures — a common panel footprint for a single safety function.
The 24 V DC rated supply is the standard control voltage for most safety PLCs and I/O blocks — no separate power supply needed if your cabinet already runs 24 V. The push-in spring terminals (spring-loaded, no tool required for ferruled wire) accept 0.5 to 1.5 mm² stranded without ferrule, or 0.5 to 1.0 mm² with ferrule. That covers the typical field-wiring range for safety inputs and feedback loops. Start input and feedback input are both present on this unit, so you can wire a two-hand control station or monitor external contactors directly. The sensor input minimum off-time is 60 ms, which sets the minimum pulse duration the relay will recognize — relevant when using light curtains with short dark periods.
Depth is 121.6 mm — verify clearance to the gland plate or door, especially in shallow cabinets. Power loss is 2 W maximum, negligible for thermal budgeting inside a ventilated panel. Shock resistance is 10 g / 11 ms, suitable for machine-mounted enclosures on presses or conveyors.